Climate has been the secret driver of history, particularly in the preindustrial world. Empires and kingdoms rose to power when it was warm and wet and toppled when it became cool and dry...and climate might not be done guiding history.

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Researchers have long suspected a link between climatic variations and historical trends, but we generally don't know enough about historical climate conditions to say for sure. Now Swiss and German researchers have used precipitation records glean from Central European oak tree-rings - a shockingly accurate method of recording year-to-year climate shifts - to give a rough sense of the last 2,500 years of European climate.

Their findings speak to the power that climate wields in shaping human history. Although the current warming event is unlike anything we've ever seen before in terms of how severe and long-term it is, there are evidence of hot and cold periods in Europe's history that are either more severe or longer-lasting than the current climate change.

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Those events consistently track major historical shifts. Periods that were unusually warm and wet coincided with the rise of the Roman Empire and prosperity in the Middle Ages. As the climate fell into chaos between about 250 and 600 CE, so too did the Roman Empire. The obvious link between climate and these events is that preindustrial, agrarian societies were heavily dependent on the quality of their crop yields to sustain their civilizations. As such, it's unsurprising that long stretches of favorable crop conditions would line up with long periods of political stability and economic prosperity.

But that doesn't make climate's invisible guidance any less amazing, particularly when you consider its full scope. In their paper, the researchers link together over a dozen specific climatic phenomena with their corresponding historical event. Here are some of the highlights:

April/May/June precipitation was generally above average and fluctuated within fairly narrow margins from the Late Iron Age through most of the Roman Period until ~AD 250, whereas two depressions in June/July/August temperature coincided with the Celtic Expansion ~350 BC and the Roman Conquest ~50 BC...Distinct drying in the 3rd century paralleled a period of serious crisis in the Western Roman Empire marked by barbarian invasion, political turmoil and economic dislocation...Falling lake levels in Europe and Africa accompanied hemispheric-scale cooling that has been linked with an explosive, near equatorial volcanic eruption in AD 536, followed by the first pandemic of Justinian plague that spread from the Eastern Mediterranean in AD 542/543. Rapid climate changes together with frequent epidemics had the overall capacity to disrupt the food production of agrarian societies...

And climate wasn't just capable of building up and tearing down societies. The researchers argue that climatic factors were also at work in the onset of the devastating Black Death in the 1300s as well as the mass migrations of Europeans to the New World in the last few centuries:

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Wetter summers during the 13th and 14th centuries and a first cold spell ~1300 agree with the globally observed onset of the Little Ice Age, likely contributing to widespread famine across Central Europe. Unfavorable climate may have even played a role in debilitating the underlying health conditions that contributed to the devastating economic crisis that arose from the second plague pandemic, the Black Death, which reduced the Central Europe population after AD 1347 by 40-60%. The period is also associated with a temperature decline in the North Atlantic and the abrupt desertion of former Greenland settlements. Temperature minima in the
early 17th and 19th centuries accompanied sustained settlement abandonment during the Thirty Years' War and the modern migrations from Europe to America.

Although agrarian societies are far more vulnerable to sudden climatic shifts than industrial ones, the researchers close their paper by suggesting that climate can still very much affect the course of history, and that policymakers might want to consider whether their jobs will even still exist if current climate patterns go unchecked:

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Linking palaeo-demographic to climate proxy data challenges recent political and fiscal reluctance to mitigate projected global climate change, which reflects the common societal belief that civilizations are insulated from variations in the natural environment. The historical association between European precipitation and temperature variation, population migration and settlement desertion, however, questions the wisdom of this attitude.